1. Field of the Invention
The present invention relates to a material capable of readily developing natural patina and to a process for producing the same. More particularly, the present invention relates to a material which develops uniformly, in a short time, natural patina having excellent corrosion resistance and which is useful as building materials such as for roofing, as well as, to an easy and inexpensive process for producing such material.
2. Prior Art
Copper has been long used as building materials such as for roofing because of excellent durability and fire resistance, lightness in weight, ease of machining and freedom from maintenance after working. Today, copper finds wide applications ranging from public buildings, shrines and temples to private residences, partly because copper is available inexpensively with least fluctuation of price.
The reason why copper is used willingly as a building material is, in addition to these excellent properties inherent in copper as described above, that if copper is left to stand under atmospheric conditions for a long time, a patina film is formed spontaneously on the copper surface, and the patina film serves as a protective film to enhance corrosion resistance of the material and also adds dignified and beautiful appearance to the material.
The greatest part of natural patina is a basic copper sulfate (CuSO.sub.4.3Cu(OH).sub.2), and only in some special cases a basic copper chloride and a basic copper carbonate are found.
However, it takes usually from about more than 10 years to about 20 years for the natural patina to grow thick enough to acquire appreciable good appearance. For such reason, there have been proposed various methods for artificially developing patina on the copper surface.
For example, Japanese Patent Publication No. 12117/1980 discloses a method of developing artificial patina on the surface of a copper plate by subjecting the copper plate to anode oxidation in an electrolyte containing HCO.sub.3.sup.- or CO.sub.3.sup.2- and NaOH or KOH.
Meanwhile, Japanese Patent Publication No. 9270/1981 discloses a method of developing artificial patina on the surface of a copper plate by contacting the copper plate with a liquid chemical conversion treatment containing hydrochloric acid, copper acetate, a basic copper carbonate, copper nitrate, ammonium chloride or sodium chloride.
Japanese Patent Publication No. 30396/1981 discloses a method in which a resin coating containing a copper powder is applied on the copper surface, followed by a chemical conversion treatment using a liquid treatment containing hydrochloric acid, copper acetate, copper carbonate, arsenious acid, ammonium chloride, copper nitrate or water.
Further, Japanese Unexamined Patent Publication No. 139467/1980 proposes a method of developing aeruginous color by applying a coating incorporated with a basic copper carbonate powder, a kind of patina, on the surface of a basis material. Japanese Patent Publication No. 19910/1987 proposes a method in which an aqueous solution or suspension containing a water-soluble or water-dispersible resin and a compound which forms a complex salt with copper ion is applied on the copper surface.
However, the artificial patina formed according to the above-described methods is, in most cases, of the basic copper carbonate (in the case of anode oxidation) or a basic copper chloride (in the case of chemical conversion treatment), and these methods are hardly capable of forming the basic copper sulfate, the major component of natural patina, on an industrial scale.
Thus, composition of the artificial patina formed according to any of the above-described methods is greatly different from that of natural patina, so that the artificial patina undergoes discoloration or fading with time to lose its decorative savor as patina and also lifts off from the basis material. Besides, copper plates having such artificial patina are expensive.
For such reasons, it has been practiced still, for example in the case of roofing material, to use copper plates as such, instead of using expensive artificially patinated copper plates, to wait until the plates are spontaneously coated with natural patina. However, acid rain falls occasionally in the recent polluted atmospheric condition, so that the period required for development of natural patina is said to be much longer than before. In the case of a private residence, there is a problem that, even if the roof is coated with natural patina after a long period to present appreciable good appearance, the residence as a whole are outworn at that time and needs reconstruction, unfortunately. Accordingly, advent of a material which can develop natural patina at an early stage after roofing is earnestly desired.